This invention relates to signal processors for radar, sonar and like signal reception systems, and more particularly to sidelobe cancellation circuits for improved reception in the presence of jamming, interference and other noise signals as received by such systems.
Still more particularly, the present invention constitutes a refinement of the canceller correlation signal weighting techniques which are the subject of the copending application of Jureller and Soule, Ser. No. 488, 395 filed Jul. 15, 1974, of common assignment herewith. The invention of that application is in turn a refinement of the basic intermediate frequency sidelobe canceller circuit disclosed and claimed in U.S. Pat. No. 3,202, 990, which issued Aug. 24, 1965 to Paul W. Howells, and may embody also controlled saturation of the cancellation loop as disclosed in the copending application of Cavelos, Ser. No. 482, 078, filed Jun. 24, 1974, the patent and application being both also of common assignment herewith.
As more fully described in the aforementioned Jureller-Soule application, that invention affords improved capabilities against jamming signal inputs by isolating the cancellation loop against input of non-jammer signal energy such as attributable to clutter or to transmitted pulse energy which is cross-coupled between the transmitter and receiver. Such isolation of the cancellation loop avoids the compromise of its operation which could result from the necessity to accommodate clutter and other noise input, thus enhancing the loop's capabilities against jamming. In accordance with the Jureller-Soule invention, cancellation loop isolation is accomplished by interposing within the loop a "sample-and-hold" switching device which first makes and then breaks the loop at that point, to hold whatever correlation signal weighting exists at the end of each of the sampling periods thus established.
Such sampling periods preferably are synchronized with respect to the pulse transmitter, so as to occur at a time within the interpulse period at which the interference signal input to the canceller loop is primarily of jammer origin and comprises little if any clutter or other non-jammer interference content. In this way the canceller loop's capabilities against jamming signal input are preserved uncompromised by efforts of the loop also to cancel clutter and the like, for which other remedies are known and available.
In implementing cancellation circuits in accordance with the Jureller-Soule invention, the sampling period normally is made of duration sufficiently long to permit the canceller loop more than adequate time to lock onto the jamming signal input and to settle or stabilize the loop before the sampling switch is opened to hold the correlation signal weighting thus established in the loop. While sample-and-hold switching in this manner provides canceller loop operation which is fully adequate for most applications, it is the purpose of the present invention to further improve canceller loop capabilities in systems using sample-and-hold switching of this kind, to provide enhanced cancellation and also to reduce the transient or lock-on period during which some portion of the jamming signal will remain uncancelled by the canceller. These improvements in cancellation ratio and loop lock-on time in accordance with the present invention are accomplished with little if any additional circuit complexity and at little or no. additional cost in implementation, as will become apparent from their description hereinafter.